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Dating of sedimentary rocks

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Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, varying from early Archaean to Mesozoic in age.The formation of diagenetic xenotime is principally related to redox cycling of Fe-oxyhydroxides and microbial decomposition of organic matter, leading to elevated concentrations of dissolved phosphate and rare earth elements (REE) in sediment pore-waters.Recent advances in the field of geochronology have led to a greater understanding of the scale and duration of geological processes.It is currently possible to date igneous and metamorphic rocks by a variety of radiometric methods to within a million years, but establishing the depositional age of sedimentary rocks has remained exceedingly difficult.Local relationships on a single outcrop or archaeological site can often be interpreted to deduce the sequence in which the materials were assembled.This then can be used to deduce the sequence of events and processes that took place or the history of that brief period of time as recorded in the .These are often characterised as the norm, rather than the exception.

To get to that point, there is also a historical discussion and description of non-radiometric dating methods.

Geochronologists do not claim that radiometric dating is foolproof (no scientific method is), but it does work reliably for most samples.

It is these highly consistent and reliable samples, rather than the tricky ones, that have to be falsified for "young Earth" theories to have any scientific plausibility, not to mention the need to falsify huge amounts of evidence from other techniques.

In the case of Rb Sr dating of sediments, this rests on the assumption that Sr isotope systematics in the rock were homogenized during deposition or early diagenesis, and thereafter remained as a closed system until the present day.

However, we will see that these two requirements may be mutually exclusive.

Xenotime has the properties of an ideal U–Pb chronometer, containing elevated levels of U (generally 1000 ppm) and very low concentrations of initial common Pb.

In addition, it has an exceptional ability to remain closed to element mobility during later thermal events, and commonly yields concordant and precise dates.

his document discusses the way radiometric dating and stratigraphic principles are used to establish the conventional geological time scale.

It is not about the theory behind radiometric dating methods, it is about their , and it therefore assumes the reader has some familiarity with the technique already (refer to "Other Sources" for more information).

The example used here contrasts sharply with the way conventional scientific dating methods are characterized by some critics (for example, refer to discussion in "Common Creationist Criticisms of Mainstream Dating Methods" in the Age of the Earth FAQ and Isochron Dating FAQ).

A common form of criticism is to cite geologically complicated situations where the application of radiometric dating is very challenging.